This invention relates to non-aqueous, active metal-liquid depolarizer/electrolyte solvent-based primary battery cells, for example, lithium-thionyl chloride (Li—SOCl2) batteries. In particular, the invention relates to the alleviation of voltage delay upon load of such battery cells.
Much effort has been expended in the development of high energy density cell systems which provide both high voltage and total capacity. The high energy density cell systems are centered around the use of active metals (alkali and alkaline earth metals which are unstable in aqueous environments) as anodes in non-aqueous liquid depolarizer/electrolyte solvent-based (also often referred to as liquid cathode) cells. Lithium in particular has shown great promise as an anode material because of its high potential and low weight.
Various cell systems have been developed utilizing lithium as the anode electrode material. Cells having liquid cathode depolarizers, which generally serve the dual function of electrolyte salt solvent and cathode depolarizer, have shown promise in terms of voltage stability and high discharge capability. When a cell of this type is not being discharged, the liquid depolarizer/electrolyte solvent reacts with the anode metal to a limited extent resulting in a relatively thick resistive surface film being formed on the surface of the anode. In Li—SOCl2 cells this resistive surface film is composed mostly of polycrystalline LiCl. Full reaction between the anode and liquid depolarizer with which it is in contact is thereby substantially prevented and cell self discharge is limited. Such cells are nevertheless operable since the resistive LiCl film is dissipated during cell discharge. However, there is generally a lengthy “voltage delay” (defined as the length of time to attain an operating voltage after the initial load is applied) upon the onset of cell discharge as the film is dissipated.
One of the most common liquid depolarizer/electrolyte solvents is thionyl chloride (SOCl2) which, in combination with lithium, provides a cell couple having a very high voltage (about 3.65 V), discharge capability, energy density, and stability of discharge voltage. However, a limiting factor in the utility of Li—SOCl2 cells, particularly for applications requiring a high discharge rate, is the aforementioned “voltage delay” resulting from the formation of the resistive LiCl film on the anode. This negative effect is especially pronounced for aged cells discharging at low temperature.
Electrolyte salts or solutes used in thionyl chloride depolarized cells are well known, for example as described at length in U.S. Pat. No. 3,926,669. The most preferred and the most widely used electrolyte salt or solute is lithium tetrachloroaluminate (LiAlCl4). In order to alleviate the effects of voltage delays, the use of electrolyte salts other than LiAlCl4 has been proposed. For example, U.S. Pat. Nos. 4,238,552 and 4,020,240 describe the use certain halo-gallate, indate or thallate salts and chloroborate salts, respectively. Other alternative electrolyte salts have also been proposed. While advantages have been reported with the use of these alternative electrolyte salts, drawbacks have also been noted in at least some cases, including increased cost and/or decreased performance.
Accordingly, it is believed that Li—SOCl2 and like cells may be improved and such improvement would be desirable.